1. Field of the Invention
The invention relates to an image processing apparatus and, more particularly, to an image formation apparatus for gradation converting image data including periodicity into data in a print format of printer of an electrophotographic system.
2. Description of the Related Art
In recent years, performance of image apparatuses for inputting, displaying, and outputting an image has remarkably been improved. As typical examples, there are a 12-bits/pixel input scanner, a 10-bits/pixel display, and a 4-bits/pixel output printer.
In such image apparatuses, since image data is handled by a data format adapted to gradation reproducing ability of each apparatus, for example, in the case where the image data inputted by the scanner is printed by the printer, it is necessary to convert the number of gradations of the image inputted by the scanner into the number of gradations which can be reproduced by the printer (hereinafter, such a conversion is referred to as a gradation conversion).
For example, in a printer dependent on an electronic photographic system, a gradation conversion system called a dither method is used. As shown in FIG. 10, according to the dither method, by using a dither matrix in which (Nxc3x97N) pieces of threshold values are arranged, the threshold values are compared with an image signal, thereby determining ON/OFF for dots.
Since the number of reproducing gradations of the apparatus such as scanner or monitor for inputting the image data to the printer is generally larger than that of the printer, a density of ON/OFF of the dots is locally adjusted by using the dither matrix and the number of gradations of the input image is falsely expressed. Such a dither matrix is classified mainly into a dot concentration type in which the dots concentrate regularly by gradually increasing the threshold value from the center of the matrix and a dot distribution type in which the threshold values are arranged at random.
However, according to the electronic photographic system printer, for example, in the case where the number of output gradations is equal to a binary value (black or white), if the image data including isolated dots such that one pixel is a black pixel and eight peripheral pixels are white pixels is supplied, toner is hard to be stably adhered onto a photosensitive material and a portion of the isolated dot cannot be printed, so that there is a problem such that the gradation of the input data is not reflected to a printed result. Therefore, the dither matrix of the dot concentration type such that the black pixels concentrate on two or four pixels is used.
As an apparatus to which image apparatuses of different gradation reproducing abilities are connected, there is a copying apparatus in which a scanner and an electronic photographic system printer are implemented as shown in FIG. 11. Such a copying apparatus is used for printing data from a personal computer or copying printed matter. Most of the printed matters as targets of the copy have a period of a density change of a light/dense pattern as shown in FIG. 12. It is based on an image format according to printing characteristics or the number of reproducing gradations of the printer and is peculiar to the printer.
In the case where the printed matter having such a period of the light/dense pattern is read by the scanner and gradation converted for use in the electrophotographic system printer, a moire occurs and deterioration in picture quality is caused. The moire is caused by interference between the period of the light/dense pattern included in the printed matter and the period of the light/dense pattern which is formed by the dither matrix of the gradation conversion system.
As one of methods of suppressing such a moire, there is a method disclosed in JP-A-4-104576. According to this method, the periodicity of printed matter is lightened by adding a low pass filter to the front stage of the gradation conversion. However, there are variety of periods as a period included in the printed matter, and in order to obtain a good result for all printed matter, the period included in the printed matter has to be discriminated and coefficients of the low pass filter have to be changed. There is, consequently, a problem such that a scale of circuit which is used for suppressing the moire is enlarged and a load of processes also increases.
As another method of suppressing the moire, as disclosed in JP-A-7-123259, there is a method whereby an error diffusing method is applied to the gradation conversion system. The error diffusing method is a gradation conversion system such that a process of propagating quantization errors caused by quantization based on a threshold value to data which is not quantized is executed on a pixel unit basis, thereby realizing the gradation reproduction of high precision. According to the error diffusing method, however, since isolated dots which cannot be printed by the printer of the electronic photographic system are formed particularly in a low density portion, it is necessary to suppress the isolated dots by combining a template process and other processes. There is also a problem such that in order to prevent causes of deterioration in the picture quality such as a chain-like texture that is peculiar to the error diffusing method, a circuit scale is enlarged and a load of processes also increases.
On the other hand, like xe2x80x9cBinarizing method by threshold matrix error diffusing method and memory mediumxe2x80x9d disclosed in JP-A-10-150565, there is also a system in which by using a blue noise mask as a threshold value in the error diffusing method, various problems of the picture quality which are peculiar to the error diffusion are improved. In a manner similar to the error diffusing method, such a system also has a problem such that isolated dots are formed and the gradation is not reflected to a print result.
In consideration of the problems as mentioned above, it is an object of the invention to provide an image formation apparatus in which in a gradation conversion system of the electronic photographic system, a moire which is caused in the case where an input image including periodicity is gradation converted is suppressed and the generation of isolated dots by which the gradation is not reflected to a print result is reduced.
Another object of the invention is to simplify the gradation converting process and realize a high processing speed.
To accomplish the above objects, according to the invention, data which is aperiodic between pixel areas having a neighboring relation is formed and converted into a plurality of threshold data for forming a systematic dot array in the pixel area, and threshold values are compared with image data. Thus, data in a print format having the systematic dot array in which a moire is suppressed between the pixel areas and the generation of isolated dots has suppressed in the pixel area can be formed.
According to the invention, a threshold value of blue noise mask which is usually made to correspond to data on a pixel unit basis is made to correspond to a pixel area and converted into a plurality of threshold data for forming a systematic dot array in the pixel area, and the threshold values are compared with image data. Thus, data in a print format having the systematic dot array in which a moire is suppressed between the pixel areas and the generation of isolated dots has been suppressed in the pixel area can be formed.
According to the invention, a dither matrix constructed by n numbers of threshold data for forming a dot array which is aperiodic between threshold value areas and systematic in the threshold value area is used. Thus, data in a print format having the systematic dot array in which a moire is suppressed between the threshold value areas and the generation of isolated dots has been suppressed in the threshold value area can be formed.
According to the invention, a threshold value of a blue noise mask is made to correspond to a pixel area, a dither matrix in which the threshold value of the blue noise mask is converted into a plurality of threshold data for forming a systematic dot array in the pixel area and the plurality of threshold data is arranged is used, and the threshold values are compared with image data. Thus, data in a print format having the systematic dot array in which a moire is suppressed between the pixel areas and the generation of isolated dots has been suppressed in the pixel area can be formed.
According to the invention, a conversion error which is caused when data in a print format is determined by comparing image data with threshold data is propagated to the image data which is not quantized on a pixel area unit basis or on a threshold value area unit basis. Thus, a moire which is caused by a gradation converting process based only on the threshold value comparison is suppressed and, further, isolated dots which are caused by the propagation of a quantization error of a pixel unit can be suppressed.
According to the invention, a correspondence relation between the aperiodic data or the threshold value of the blue noise mask and the threshold value which is made to correspond to each pixel in the pixel area is preliminarily stored as a table, thereby making a calculation for obtaining the threshold values unnecessary. The threshold data can be obtained at a high speed.
According to the invention, the systematic dot array is a threshold value array such that dots concentrate in the pixel area or the threshold value area due to a decrease in luminance value or an increase in density value of the image data.
According to the invention, for example, in the case where the aperiodic data is assumed to be TH and the number of pixels in the pixel area or the number of threshold data in the threshold value area is equal to N, a plurality of threshold data to form the systematic dot array in the pixel area or the threshold value area is the aperiodic data and the threshold data determined by the number of pixels in the pixel area or the number of threshold data in the threshold value area in a manner such that the threshold value which is used for quantization of the first pixel is equal to 1xc2x7TH/N, the threshold value which is used for quantization of the second pixel is equal to 2xc2x7TH/N, and the threshold value which is used for quantization of the Nth pixel is equal to Nxc2x7TH/N, respectively.
Or, for example, in the case where the threshold value of blue noise mask is assumed to be TH and the number of pixels in the pixel area or the number of threshold values in the threshold value area is equal to N, a plurality of threshold data to form the dot array which is systematic in the pixel area or the threshold value area is the threshold value of blue noise mask and the threshold data determined by the number of pixels in the pixel area or the number of threshold values in the threshold value area in a manner such that the threshold value which is used for quantization of the first pixel is equal to TH/N, the threshold value which is used for quantization of the second pixel is equal to 2xc2x7TH/N, and the threshold value which is used for quantization of the Nth pixel is equal to TH, respectively.
According to the invention, for example, in the case where the number of pixels in the pixel area or the number of threshold values in the threshold value area is equal to n and quantization errors of the image data corresponding to the pixel areas or the threshold value areas are assumed to be El to En, in the correction value determination unit, the quantization error in the pixel areas or the threshold value areas is the sum (xcexa3En) of them, and the correction value is determined to be 1/nxc2x7xcexa3En.
Further, according to the invention, there is provided a method of forming a dither matrix, wherein each threshold value of blue noise mask is made to correspond to a threshold value area comprising a plurality of threshold data having an interlocking relation, and the threshold value of the blue noise mask is converted into a plurality of threshold data for forming a systematic dot array in the threshold value area, thereby forming the dither matrix.
Further, according to the invention, there is provided a copying apparatus having a scanner unit, a printer unit, and a gradation converting unit for converting image data read by the scanner unit into print data to be printed by the printer unit, wherein the foregoing image formation apparatus is applied to the gradation converting unit.
According to the invention, there is provided an image formation apparatus for converting image data including periodicity into data in a print format of a printer of an electronic photographic system, wherein threshold data for forming a systematic dot array in a pixel area comprising a plurality of pixels having an interlocking relation is formed from data which is aperiodic between the pixel areas. Thus, it is possible to obtain a gradation conversion result in which the generation of a moire between the pixel areas is suppressed and which has a systematic dot array such as dot concentration or the like in the pixel area.
The propagating process of propagating the quantization error caused by the comparison of the image data and the threshold data to the image data is executed on a unit basis of a pixel area for forming the systematic dot array. Thus, as compared with the case of performing the propagating process on a pixel unit basis, an amount of signal processes can be reduced and a data capacity which is used for accumulating the quantization errors can be also reduced. Further, the moire which is caused in the gradation converting process based only on the threshold value comparison can be suppressed, and the isolated dots which are generated by the error propagation of the pixel unit can be suppressed.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.